A rechargeable, current producing, electrochemical cell has to satisfy many requirements in order to be of practical value. Among the requirements is the capability to operate efficiently at elevated temperatures for many discharge charge cycles.
One attractive class of modern high energy density cells makes use of alkali metal anodes, non-aqueous electrolytes and transition metal sulfide cathodes. The latter are solid compounds which upon reduction incorporate the alkali metal without fundamental structural changes. Examples of such cathode materials are TiS.sub.2, TiS.sub.3, MoS.sub.2, MoS.sub.3, NbS.sub.2, NbS.sub.3, V.sub.2 S.sub.5, and V.sub.x Cr.sub.1-x S.sub.2. Typical electrolytes include dioxolane, tetrahydrofuran, dimethoxy ethane, and mixtures thereof with LiAsF.sub.6 or other lithium salts. The most commonly use anode is Li or a Li alloy. A specific example is a lithium (Li)/2 methyl-tetrahydrofuran (2Me-THF)-tetrahydrofuran (THF)-Lithium hexafluoroarsenate (LiAsF.sub.6)/Titanium disulfide (TiS.sub.2) cell for which the reaction can be written as follows: EQU xLi+TiS.sub.2 .fwdarw.Li.sub.x TiS.sub.2 (0&lt;x&lt;1) E.about.2.1 V
Since TiS.sub.2 incorporates Li without fundamental structural changes one expects that such a cathode can be charged and discharged many times with little change in capacity. Furthermore, to the degree that mass transport processes in the electrolyte and/or the cathode limit cell performance, one would expect performance to improve as the operating temperature is increased. However, in practical batteries this expected improvement is often offset by undesirable side reactions. Such side reactions occur in secondary lithium/transition metal chalcogenide cells and they result in markedly shortened cycle life at elevated temperatures.
Cells consisting of a Li anode, a TiS.sub.2 cathode and a 2MeTHF/THF/2MeF/LiAsF.sub.6 electrolyte show upon initial discharge almost complete cathode reduction, i.e., formation of Li.sub.x TiS.sub.2 where x.about.1. In these cells the anode material is provided in excess to the stoichiometric amount needed for cathode reduction. Thus cell performance, is at least initially, determined by the cathode, although the ultimate cycle life may be limited by the anode.
Upon discharge-charge cycling at room temperature (.about.25.degree. C.), cathode utilization decreases gradually from about 90% in early cycles to .about.70% after 80 cycles. Similar test cells cycled at 65.degree. C. degrade in performance much earlier in cycle life. Cycle life at 65.degree. C. is only 12 cycles to 70% cathode utilization.
While the performance described above is typical, it is well known to persons skilled in the art that the exact performance of a cell depends on many parameters including cathode structure, cell assembly and test conditions. However, a similar substantial degradation of cycle life is typically observed at elevated (65.degree.-70.degree. C.) temperatures.